The present invention is directed to communication systems, and is particularly directed to a test unit resident mechanism for automatically deriving, from a single ended wireline access point, the location and amplitude of a source of noise that may impair digital communications along a wireline telecommunication link.
In the face of the increasing demand for a variety of digital communication services (such as, but not limited to internet services), telecommunication service providers are continually seeking ways to optimize the bandwidth and digital signal transport distance of their very substantial existing copper plant, that was originally installed to carry nothing more than conventional analog (plain old telephone service or POTS) signals. In addition to the inherent bandwidth limitations of the (twisted pair) copper wire medium, service providers must deal with the fact that in-place metallic cable plants are subject to a variety of influences, such as RF signals emitted by a local radio station, and cross-talk from one or more adjacent twisted pairs, that can introduce data-impairing noise into a data transporting wireline path.
Although currently available (remote) test units allow telco service personnel to conduct a number of different electrical measurements on a line under test (LUT), such as but not limited to variable tone measurements (e.g., over a sweep frequency of up to 1.5 MHz), bridge tap detection, capacitance measurements, and the like, these units are essentially single ended devicesxe2x80x94being connectable to only a single point of access of the wireline link, typically at or in the immediate vicinity of a central office""s main distribution frame.
This single ended connectivity limitation has conventionally constituted a major impediment to identifying and locating noise sources in an existing copper plant, such as one that needs to be evaluated for digital data transport capability. In a standard (single ended) test unit, the noise source cannot be accurately identified, since the test head cannot distinguish between a relatively weak noise signal introduced very close to the point of access, head, or a very large amplitude source injected much farther (for example thousands of feet) away (for example at a customer premises site), and attenuated by associated cable loss characteristics.
This inability of conventional single ended measurement schemes to accurately and reliably identify the location and amplitude of a noise source along a wireline telecommunication link is effectively obviated by means of a wide band noise extrapolation mechanism of the present invention, which may be readily implemented in a processor-controlled test head installed in a central office, or as part of test signal generation and processing circuitry of a portable craftsperson""s test set.
Pursuant to the invention, the test head contains test signal generation and processing circuitry, that is operative to process a prescribed set of wireline measurement data, including noise and cable plant parameter measurements conducted from a single access point, to reliably identify both the location and the amplitude of a noise source along any of a plurality of wireline cable plants that extend from an access point to various drop sites, where the cable plant is most susceptible to noise ingress. As will be described, the invention employs a measurement analysis mechanism that combines the ability to make accurate electrical measurements from a single access point with the inherent characteristics of the cable plant, to extrapolate both the distance of the noise source from the test point, as well as the actual amplitude of the noise as injected or induced at the remote source.
In the non-limiting example to be detailed below, the invention resides in a processor-controlled test head, that is selectively connectable to respective cross connect points of an interconnect matrix switch, such as, but not limited to a CTAS interconnect matrix switch, employed by a central office to connect high data rate digital data traffic channels supplied by a digital subscriber loop access multiplexer with selected wireline twisted pairs. While some of the wireline connection points of the (CTAS) matrix switch may be effectively unterminated, the ones of interest will be coupled by a multidistribution frame to subscriber loop xe2x80x98dropsxe2x80x99, that may extend a substantial distance out to a customer premises equipment site.
Pursuant to the automated single ended noise measurement and location mechanism of the mechanism a noise measurement is conducted by the test head for each cross-connect matrix point of the matrix switch, so as to derive a noise value for each wireline. Each measured noise value is stored in association with its cross-connect point, so that it may be associated with any cable plant that extends from the access point. Unterminated cross points are those that have no wirelines at all, or have extremely short pairs, that are visibly unterminated. Since unterminated points of the matrix switch are connected to no remote location, any noise measured at these points represents noise sourced in the central office, and is considered to be the lowest amplitude noise value. These lowest noise measurements are averaged to derive a noise background value, that will be subtracted off the noise values associated with remote cable plants.
The wireline pairs that extend to various CPE sites include intermediate length sections between the matrix switch and the main distribution frame, and considerably longer sections of cable plant to the CPE sites. Due to strict engineering practices at telephone company facilities, and the fact that its components are protected and not easily accessible by field personnel, the likelihood that significant noise sources are associated with the central office is extremely low. On the other hand, as the wireline cable plants to CPE sites are mostly unshielded, untwisted cable, that is high susceptible to noise, any substantial noise may be validly assumed to be sourced at or in the vicinity of the CPE sites. The lengths of the cable plant to the CPE sites may be readily estimated using capacitance measurements conducted for each cross-connect point. In addition, the attenuation for the estimated lengths of cable plant may be determined using industry standard copper loop attenuation characteristics.
Each respective noise measurement value is compared with a prescribed threshold indicative of what is considered to be a xe2x80x98failurexe2x80x99 of the line, as may be due, for example, to excessive crosstalk from another digital communication service, or as a result of a physical impairment on the line, that requires physical intervention (e.g., removal and/or repair) by service personnel. If the noise threshold is exceeded, the line is marked for service by telco personnel, so that the problem may be resolved, and thereby improve the line""s digital transport capability.
The locations of those noise values that do not exceed the failure threshold are determined using the cable plant length estimates. These estimates may be validly employed, since those portions of the wireline pairs which are most susceptible to noise are the generally unshielded, untwisted drops, that extend to the CPE sites. This implies that the actual distance of the noise source from the single ended measurement point of the test head at the interconnect matrix switch is the same as the cable plant length for the test point of interest.
The actual amplitude of the noise source whose location has been identified is estimated by subtracting the background value, from the original noise measurement to produce an xe2x80x98adjustedxe2x80x99 noise value that is attributable to only the noise source at the cable drop source, but which has been attenuated by the cable plant between the drop site and the measurement point at the matrix switch. Since the cable length and loss are known, the actual noise amplitude for a respective wireline is equal to the adjusted noise amplitude multiplied by the estimated cable length and the loss per unit length of the wireline.